New Auburn - With four sand mines in operation within a few miles of New Auburn public schools, district officials have taken extra precautions to keep silica sand out of the building's air system. Superintendent Brian Henning says the district has been using higher-quality air filters at double the price, spending about $1,500 this school year. West-central Wisconsin is a hot spot for sand mining and residents have expressed environmental and health concerns about the tiny sand particles generated by mining. The sand is mixed with chemicals in hydraulic fracturing _ a high-pressure process used to extract natural gas and oil from the ground. Henning tells the Eau Claire Leader-Telegram (http://bit.ly/HjCgBE ) school officials took dust scrapings from the air filters and sent it to a Madison lab for testing. They found a small amount of silica on the filters.